The one that sounds the best. Generally it's stereo, but it depends on the sound, context, mix and playback device(s). As pointed out below, music and ambiences are almost always stereo, but in terms of SFX and dialogue it's more of a matter of taste and depends on the processes that have been applied as well (e.g. a stereo reverb). If the playback device is mono then playing back stereo audio is worthless and likely sounds bad because of the phasing that results from downmixing the channels, unless the file and/or the mix is assured to be closely phase coherent / "mono-compatible". Note that on mobile devices people may be using the internal speaker(s) (likely a mono speaker) or headphones (stereo), so you need to strike a balance between for which playback you'll optimize more or whether you want to optimize for both. Reference monitoring is important.
The game engine may also affect the choice of channel counts in particular sounds (e.g. 3D positional audio works only with mono files AFAIK). Or there may be other specific restrictions, see the documentation.
And you may need to produce mono files just for the sake of getting a smaller file size, if you might end up using too much space.